1. Field of The Invention
The present invention relates to a copier which forms a latent electrostatic image on a recording medium in correspondence with the image of a document, develops the latent image with developer, transfers the developed image to a copy paper and fixes the image.
2. Related Art
In a copier which makes use of electrophotographic principles, units for charging, exposing, developing, transferring, discharging, cleaning and others are usually arranged in this order around a photoconductor as a recording medium.
Because units for the above-mentioned process steps are arranged around the photoconductor, the diameter of the rotary drum which supports the photoconductor cannot be made small, and the positions of the units are restricted.
A different kind of process called a two-rotation process has been applied to a copier as disclosed in Japanese patent publication No. 21706/1982. In this process, a single unit serves for both developing and cleaning. In the first rotation of the photoconductor the unit develops a latent electrostatic image formed on the photoconductor, while in the second rotation it removes toner and charge from the surface of the photoconductor at the same positions as that of developing the image.
An advantage of the two-rotation process is that number of the units can be decreased by one and the positions for the units can be determined with less restrictions.
Furthermore, both charging and transferring can be assigned to a single unit in this process. The photoconductor is charged homogeneously with a specified polarity in the charging process. Then, an optical image is exposed to form a latent image on the photoconductor. The latent image is developed by toner which has been charged for example by friction charging with a polarity opposite to the above-mentioned specified polarity. Then, the developed image is transferred to a copy paper charged with the appropriate specified polarity so as to transfer the toner thereto. The polarity of the charges both in the charging and transferring step is the same for each. This makes it possible to assign a single unit to both charging and transferring. Accordingly, the number of the units can be decreased further by one. A copier wherein the number of units is decreased as mentioned above has already been developed.
However, the assignment of charging and transferring to a single unit results in the following disadvantages. First, because the unit should be arranged after the unit for developing, the distance between the charging (transferring) position and the exposing position becomes necessarily long. Therefore, the dark decay after the homogeneous charging becomes large such that when the photoconductor arrives at the exposing position it becomes difficult to form a correct latent electrostatic image, and the contrast of the image decreases. Therefore, a photoconductive material having a low dark decay property should be used for a photoconductor. Second, the arrangement of a corona charging device for transferring becomes inappropriate, that is, since a copy paper which is usually fed horizontally is charged from the rear side thereof in the transferring process, the aperture of the corona charging device is opened upwardly in order to charge the copy paper. Then, a portion of the toner deposited on the photoconductor after the development falls into the corona charging device through the aperture thereof to soil the corona charging line. This invites serious irregular charging.
These disadvantages are related closely with the fact that a single unit is used both for charging and transferring, and they are difficult to overcome.